YSI 6 series User Manual page 267

Principles of Operation Section 5
Light Source
Green LED
Photodetector
Optical Filter
Optical Fiber
Field optical measurements are particularly susceptible to fouling, not only from long term build up of
biological and chemical debris, but also to shorter term formation of bubbles from outgassing of the
environmental water. These bubbles can generally be removed in short term sampling application by
simply agitating the sonde manually. However, for studies longer than a few hours where the user is not
present at the site, the quality of the data obtained with a Rhodamine WT sensor that has no capability of
mechanical cleaning may be compromised. Like all YSI optical probes described above, the 6130
Rhodamine WT probe is equipped with a mechanical wiper that makes it ideal for unattended applications.
The wiper can be activated in real-time during discrete sampling operations or will function automatically
during long term unattended sampling studies. The number of wiper movements and the frequency of the
cleaning cycle for the unattended mode can be set in the sonde software. Generally one movement is
sufficient for most environmental application, but in media with particularly heavy fouling, additional
cleaning cycles may be necessary.
CALIBRATION AND EFFECT OF TEMPERATURE
The sonde software offers the option of 1 point, 2 point, or 3 point calibrations procedures. For most
applications, a 2-point calibration at 0 and 100 ug/L is sufficient. However, for maximum accuracy over
the entire 0-200 ug/L range of the sensor, a 3-point calibration procedure can slightly enhance the accuracy
of the sensor. Note that YSI does not offer Rhodamine WT standards, but does suggest a vendor for user-
production of standards and, later in this section, provides instructions for preparing a solution which is 100
ug/L in Rhodamine WT.
While the effect of temperature on the Rhodamine sensor and electronics is small, the fluorescence of
Rhodamine WT changes significantly with temperature. The combination of the two factors is
automatically taken into account by the sonde software providing temperature compensated readings.
YSI Incorporated Environmental Monitoring Systems Manual 5-29